HOL0010.1177/0959683615612563The HoloceneAlsos et al. 612563research-article2015

Research paper

The Holocene 2016, Vol. 26(4) 627­–642 Sedimentary ancient DNA from Lake © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav Skartjørna, Svalbard: Assessing the DOI: 10.1177/0959683615612563 resilience of arctic flora to hol.sagepub.com Holocene climate change

Inger Greve Alsos,1 Per Sjögren,1 Mary E Edwards,2,3 Jon Y Landvik,4 Ludovic Gielly,5,6 Matthias Forwick,7 Eric Coissac,5,6 Antony G Brown,2 Leif V Jakobsen,5 Marie K Føreid1 and Mikkel W Pedersen8

Abstract Reconstructing past vegetation and species diversity from arctic lake sediments can be challenging because of low pollen and plant macrofossil concentrations. Information may be enhanced by metabarcoding of sedimentary ancient DNA (sedaDNA). We developed a Holocene record from Lake Skartjørna, Svalbard, using sedaDNA, plant macrofossils and sediment properties, and compared it with published records. All but two genera of vascular plants identified as macrofossils in this or a previous study were identified with sedaDNA. Six additional vascular taxa were found, plus two algal and 12 bryophyte taxa, by sedaDNA analysis, which also detected more species per sample than macrofossil analysis. A shift from Salix polaris-dominated vegetation, with Koenigia islandica, Ranunculaceae and the relatively thermophilic species Arabis alpina and Betula, to Dryas octopetala-dominated vegetation ~6600–5500 cal. BP suggests a transition from moist conditions 1–2°C warmer than today to colder/drier conditions. This coincides with a decrease in runoff, inferred from core lithology, and an independent record of declining lacustrine productivity. This mid-Holocene change in terrestrial vegetation is broadly coincident with changes in records from marine sediments off the west coast of Svalbard. Over the Holocene sedaDNA records little floristic change, and it clearly shows species persisted near the lake during time intervals when they are not detected as macrofossils. The flora has shown resilience in the presence of a changing climate, and, if future warming is limited to 2°C or less, we might expect only minor floristic changes in this region. However, the Holocene record provides no analogues for greater warming.

Keywords ancient DNA, Arctic, climate change, metabarcoding, plant macrofossils, vegetation reconstruction Received 26 March 2015; revised manuscript accepted 21 September 2015

Introduction Future global warming is expected to be strongest in the Arctic, Pollen analyses of arctic sediments can show compositional with summer temperatures likely 2–4°C (or more) higher than changes in herb-dominated tundra vegetation (e.g. Cwynar, 1982; today and sea-ice cover drastically reduced (Collins et al., 2013; Fredskild, 1973), but palynologists must deal with the low pollen Xu et al., 2013). Our understanding of likely responses to future concentrations that result from low pollen productivity (Lamb environmental change is aided by studies of the past, such as the and Edwards, 1988). Indeed, in the High Arctic, pollen concentra- reconstruction of long-term vegetation dynamics and species tions may be too low to provide sufficient material for reliable diversity patterns in relation to past climate change. While most reconstructions (Birks, 1991; Rozema et al., 2006). Pollen grains arctic vegetation reconstructions to date have been based on pol- are variably resolved taxonomically, particularly in the Arctic, len and macrofossils (e.g. Bennike, 2013; Bigelow, 2013; Kien- ast, 2013), the potential of a molecular approach has recently been 1Tromsø Museum, University of Tromsø – The Arctic University of demonstrated (Anderson-Carpenter et al., 2011; Willerslev et al., Norway, Norway 2003, 2014). Analysis of sedimentary ancient DNA (sedaDNA) 2University of Southampton, Highfield Campus, UK 3 augments information on past species composition derived from University of Alaska Fairbanks, USA 4Norwegian University of Life Sciences, Norway conventional techniques (Giguet-Covex et al., 2014; Jørgensen 5University Grenoble Alpes, LECA, France et al., 2012; Pansu et al., 2015; Parducci et al., 2012b; Pawlowska 6LECA, CNRS, France et al., 2014). In the Arctic, cold conditions favour good preserva- 7Department of Geology, University of Tromsø – The Arctic University tion of material, and small floras allow the development of com- of Norway, Norway prehensive molecular reference libraries (Sønstebø et al., 2010), 8Centre for GeoGenetics, Natural History Museum of Denmark, both of which contribute to effective results. However, further University of Copenhagen, Denmark exploration of the method is required (Pedersen et al., 2015), Corresponding author: particularly in relation to lake sediments, which are important Inger Greve Alsos, Tromsø Museum, University of Tromsø – The palaeo-archives in the Arctic (e.g. Kaufman et al., 2009; Arctic University of Norway, NO-9037 Tromsø, Norway. Overpeck et al., 1997). Email: [email protected]

Downloaded from hol.sagepub.com at Aberystwyth University on June 2, 2016 628 The Holocene 26(4) where taxa in several diverse groups are barely distinguishable Holocene and includes several species outside their present below family level (e.g. Poaceae, Cyperaceae, Salicaceae). In con- geographical distribution (Arabis alpina, Salix herbacea, Harri- trast, macrofossil records can be more floristically informative, as manella hypnoides; Birks, 1991). In addition, the depositional they are often identifiable to genus or species level, and they are environment of this site is well studied (Holmgren et al., 2010; also more representative of the local vegetation (Birks, 2003; Birks Landvik et al., 1987). The main goals for this study were to and Birks, 2000). However, deposition and preservation of identifi- (1) compare taxonomic resolution, taxonomic overlap and detec- able plant remains vary considerably among species and sites. tion success of the sedaDNA and plant macrofossil records; How vegetation is represented by sedaDNA is less well under- (2) use sedaDNA data, plant macrofossil and sediment analyses to stood. Yoccoz et al. (2012) demonstrated that the amount of DNA infer past environmental change; (3) consider what Holocene (i.e. sequence abundance) in modern soil samples was related to vegetation change can tell us about the impact on vegetation of local above-ground biomass. Noisy but significant linear relation- expected future warming in the High Arctic; and (4) explore ships showed an approximate 1:1 relationship for graminoids, whether sedaDNA increases our understanding of past flora and whereas woody taxa were under-represented and forbs over-rep- vegetation when combined with other proxy approaches. resented in the DNA. These patterns may reflect the absolute amount of DNA in the soil, as determined by litter turnover rate, lignin content and root:shoot ratio (Yoccoz et al., 2012). To date, Methods we have no information as to whether such a relationship holds Study site with DNA in lake sediments. Lake Skartjørna (previously spelled Skardtjørna) is located on the Several studies based on a range of depositional environments, west coast of Spitsbergen (61 m.a.s.l., Figure 1). It is dammed by different floras and varying levels of taxonomic resolution have a prominent raised beach ridge that forms the postglacial marine compared sedaDNA and pollen. The conclusions are not readily limit at 65 m.a.s.l., and it has been cut off from the sea since its generalized; results tend to show higher taxonomic resolution in formation about 13,000 cal. BP (Landvik et al., 1987). An almost sedaDNA but more taxa identified overall in pollen. A generally circular 7.5 m deep basin east of the centre constitutes the deepest low floristic overlap between pollen and sedaDNA (Jørgensen part of the lake. The lake area is 0.10 km2 and the total catchment et al., 2012; Parducci et al., 2012b, 2013; Pedersen et al., 2013) may is 1.24 km2 (Holmgren et al., 2010). Air photos from recent indicate that sedaDNA is of local origin (Haile et al., 2007, 2009; decades show the lake level fluctuating by 1–2 m (Figure 1). The Willerslev et al., 2007), whereas in the localities studied, pollen is site was visited for coring from lake ice in March 2013 and then probably derived from the regional vegetation. Indeed, compari- again in September 2015 to record the flora and geology of the sons of sedaDNA and plant macrofossils show higher taxonomic catchment area. The catchment of the lake is located on the South overlap, which would be expected if both reflect local sources facing thrustal scarp of the junction between the Southwestern (Jørgensen et al., 2012; Parducci et al., 2012b; Pedersen et al., 2013; Basement Province dominated by metamorphic rocks (phyllites Porter et al., 2013), with one exception (Parducci et al., 2015). and quartzite) and the post Caledonian orogeny lithologies that The sedaDNA technique potentially has several advantages: a make up the bedrock slopes draining into the lake (Hjelle et al., standardized and objective mode of identification, high taxo- 1986; Ohta et al., 1991; Dallmann, 2015). The northern bedrock nomic resolution and (when techniques are well established) slopes are composed of the Neoprotozoic Løvliebreen formation more time-efficient production of results. The detailed floristic of psammo-pelitic phyllite, while the southern slope comprises information retrieved (as with macrofossils) can contribute to, for limestone with magnetite and haematite layers of the Malmberget example, biodiversity estimates and interpretations of environ- unit. The north eastern bedrock free-face also revealed a zone of min- mental changes using indicator taxa. An effective exploration of eralization and enhanced weathering. However, parts of the northern the ability of sedaDNA to reveal past plant community composi- and all of the southern bedrock slopes are obscured by ice-cored tion should ideally be based on floras that are well known and and vegetation-free moraine ridges that contain a wider range of should employ careful comparisons across levels of taxonomic lithologies including marbles, shales, siltstones, dolomitic lime- resolution. The Svalbard archipelago is an ideal study system to stone and sandstones predominantly of Carboniferous age. These investigate the potential of sedaDNA. The majority of the flora is lateral moraines also contain a sand and silt-rich matrix which available in a DNA taxonomic reference library (Sønstebø et al., contributes sediment directly into the lake. It is also pertinent that 2010), assuring reliable assignment to taxon. The number of vas- most of these slopes are steep with free faces above steep debris- cular plant species is low (176), and plant distributions, thermal cones. There is therefore in this small catchment a direct coupling requirements and geological preferences are well known (Alsos between slope conditions and sediment delivery into the lake, et al., 2015; Elvebakk, 1982, 1989). Potential modern vegetation making it highly sensitive to changes in snowmelt runoff, active analogues are well described from the archipelago (Elvebakk, layer instability and vegetation cover. Although not observable, it 1994, 2005; Klimešová et al., 2012). is likely that the upper raised beach ridge that bounds the lake to Spitsbergen, the largest island in the Svalbard archipelago, was the west is deposited on top of a terminal moraine. The axial almost completely glaciated at the Last Glacial Maximum (Hormes drainage of the catchment to the lake is from the Tjørnskaret pass et al., 2013; Ingólfsson and Landvik, 2013; Landvik et al., 1998). (Figure 1), which has formed a fan delta along the northeast shore The fjords on western Spitsbergen were deglaciated between of the lake. A small component of clastic sediment is also expected c. 14,100 and 11,200 cal. BP (e.g. Baeten et al., 2010; Forwick and to derive from lakeshore erosion and wind transport. Vorren, 2009; Hald et al., 2004; Mangerud et al., 1992). While late- The site is within the bioclimatic zone B (northern arctic tun- glacial vegetation records are lacking from this region (reviewed in dra zone, Elvebakk, 2005; Walker et al., 2005). The catchment Birks et al., 1994), a number of Holocene palaeoecological records area is characterized by polygon features and vegetation with (9000 cal. BP and onwards) are available (see Bernardova and Kos- overall only 10% cover, except locally in more stable sites where nar, 2012). Based on a plant macrofossil record from the same lake there is up to 100% cover. The vegetation mosaic comprises wet that is the subject of this study, Birks (1991) concluded that while sites dominated by bryophytes, unstable mesic sites dominated by vegetation cover has decreased over time, the flora has not changed Saxifraga aizoides and/or the trailing form of Saxifraga oppositi- substantially in the last 8000 years, suggesting long-term stability folia, stable mesic sites dominated by Salix polaris, Silene acaulis in species composition. This contention provides an interesting and Bistorta vivipara, and, locally, dry Dryas octopetala heath. target to assess using the sedaDNA approach. Other common species are Saxifraga cespitosa, Oxyria digyna, Lake Skartjørna was chosen because of the detailed plant Luzula nivalis, Luzula confusa and Papaver dahlianum. There macrofossil record mentioned above, which covers most of the are scattered occurrences of Puccinellia vahliana, Saxifraga

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plugged and taped immediately to prevent contamination. When splitting the core, some sediment was pushed out under pressure, and the lowermost 5 cm from the uppermost section were lost (section C, 166 cm below sediment surface). In order to account for this, a 5 cm hiatus was added (166–171 cm) and the applied depths of the core segment were adjusted upwards. All depths are given as centimetres below sediment surface. The core was kept at 1–10°C during transport and subsequently stored at 4°C at the UiT - The Arctic University of Norway. It was later transported to the Centre for GeoGenetics, Copenhagen University, where it was split in half. From one half, sub-samples of 8 g were taken using sterile disposable syringes. The core was returned to Tromsø, where the same core half was sub-sampled for radiocarbon dating, loss-on-ignition (LOI), grain-size analysis and plant macrofossil analysis. The second half was kept intact as a reference core and exclusively used for line-scan imaging and non-destructive x-ray fluorescence core scanning.

Radiocarbon dating and chronology Twelve samples of plant macrofossils were AMS radiocarbon dated at the Poznan Radiocarbon Laboratory of the Adam Mick- iewicz University, Poland (Table 1). Calibration was done using IntCal13 (Reimer et al., 2013), and the age–depth relationship was modelled by Bayesian statistics using the program MacBa- con 2.2 with default settings (Blaauw and Christen, 2011). The sedimentation rate was based on the age–depth relationship and rounded off to closest 0.1 mm/yr.

Lithological analyses Colour line-scan images with a resolution of approximately 70 µm were acquired using a Jai L-107CC 3 CCD RGB Line Scan Camera installed on an Avaatech XRF core scanner. The sediment surface was subsequently covered with 4-µm-thick Ultralene foil, and qualitative element-geochemical analyses were carried out with the Avaatech XRF core scanner. The mea- surements were carried out at 10 mm steps (each step covered 10 mm down-core and 12 mm cross-core). Instrument settings were 10 kV, 1000 µA, 10 s count time and no filter. Data process- ing was performed using WinAxil version 4.5.6. The results are presented as ratios of selected elements divided by the most con- servative element, Ti, to minimize the influence of water and matrix effects (Tjallingii et al., 2007; Weltje and Tjallingii, 2008). Scanning failed for a part of the core (319–341 cm), and re-scanning revealed problems with the calibration; we thus had Figure 1. The arctic archipelago of Svalbard (with the exception of the island of Bjørnøya to the south) and Lake Skartjørna. X denotes to exclude this part to avoid risks of bias. coring site. LOI and water content were measured using 10 g sub-samples every 4 cm, with intervals occasionally adjusted in relation to lithological boundaries. Samples were weighed, dried overnight svalbardensis, Micranthes tenuis, Cochlearia groenlandica, at 105°C, weighed again, combusted for 3 h at 550°C, allowed to Sagina nivalis, Poa alpina, P. pratensis, Cerastium arcticum and cool in a desiccator and re-weighed. Water content is based on the C. regelii. In the upper part of the catchment area, at Tjørnskaret, dry weight and expressed as a percentage of the wet (original) patches of 100% bryophyte cover with 10% cover of Huperzia weight. LOI is based on the post-ignition weight and expressed as arctica occur, indicating slightly warmer growth conditions. The a percentage of the dry weight (see Heiri et al., 2001). annual precipitation measured at Isfjord Radio, 12 km north of the Approximately 0.5 cm3 of sediment was sub-sampled for lake (Figure 1), is 320–470 mm (Førland et al., 2011). Mean July grain-size analysis from the same intervals as for LOI. The analy- and February temperatures in the period 1961–1990 were 4.8°C sis was carried with a Beckman Coulter Laser Particle Size Anal- and −12.4°C, respectively, with an annual mean of −5.1°C (http:// yser (LS 13320) at the geological laboratory at UiT. The results eklima.met.no). were analysed using Gradistat v8 (Blott and Pye, 2001), and pre- sented as fractions of clay (<2 µm), fine silt (2–8 µm), medium silt Sediment coring and sub-sampling (8–16 µm), coarse silt (16–63 µm) and sand (63–500 µm). Although small stones (gravel sized particles) were observed Ground-penetrating radar was used to locate the deepest sediment occasionally in the sediments, no such particles were recorded, package (77.96167° N, 13.81958°E). A 5.3 m sediment core probably because of the small sample size. (7.4–12.7 m below ice surface) was retrieved using a Nesje corer (Nesje, 1992) loaded with a 6 m long, 10 cm diameter PVC tube. The upper 65 cm of the core consisted of soft/liquid sediments DNA extraction and amplification that had to be discarded in the field. The core was split into three DNA was extracted from 40 samples and 15 negative controls sections to facilitate transport and handling. Core tops were at the ancient DNA (aDNA) dedicated laboratories at the

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Table 1. Radiocarbon dates for Lake Skartjørna shown with their 1σ error, calibrated weighted (W) mean, calibrated median and calibrated 95% confidence age ranges.

Depth (cm) Laboratory ID 14C age Cal. W. mean Cal. median Cal. 2o range Sample contents

75.0 Poz-58874 1305 ± 30 1238 1235.5 1120–1302 Salix polaris leaves, leaf frag. 135.0 Poz-58875 1930 ± 30 1884 1874.0 1781–1988 Salix polaris leaves 204.0 Poz-58870 2600 ± 30 2746 2746.0 2613–2863 Salix polaris leaves, leaf frag. 259.0 Poz-58871 3580 ± 50 3810 3902.0 3629–3970 Salix polaris leaves, leaf frag. 309.0 Poz-58872 4025 ± 30 4497 4483.0 4400–4624 Salix polaris leaves 341.0 Poz-58873 4420 ± 50 4946 5045.0 4825–5080 Salix polaris leaves, leaf frag. 375.5 Poz-65652 4560 ± 40 5337 5194.5 5186–5472 Salix polaris leaves 383.0 Poz-58865 4700 ± 160 5424 5342.0 5294–5551 Salix polaris leaves 387.0 Poz-65653 4700 ± 40 5479 5449.5 5350–5595 Salix polaris leaves, leaf frag. 434.0 Poz-58866 5650 ± 40 6421 6443.5 6271–6594 Salix polaris leaves 496.0 Poz-58868 7170 ± 40 7945 7985.5 7736–8098 Moss (Drepancladus, Cinclidium) 526.0 Poz-58869 7740 ± 40 8477 8506.5 8330–8604 Salix polaris leaves

Radiocarbon ages were calibrated (cal. BP) following IntCal13 (Reimer et al., 2013) and an age–depth model was obtained with the software Bacon (Blaauw and Christen, 2011).

Centre for GeoGenetics. For whole genome extraction, we used with only a single copy and/or shorter than 12 bp were filtered out PowerMax Soil DNA Isolation kit (MO BIO Laboratories, using obigrep. Obiclean was then used to identify amplification Carlsbad, CA, USA), following the manufacturer’s instruc- and sequencing errors, using a threshold ratio of 5% for reclassi- tions, with the exception that the centrifuge steps were done at fying ‘internal’ sequences to their relative ‘head’ sequence 4600 r/min, and samples were placed in a Fast-Prep for 2 × 20 s (Bellemain et al., 2013; De Barba et al., 2014). Finally, using at 4.0 m/s at step 2. At step 4, samples were incubated at 65°C the ecotag program (Yoccoz et al., 2012), sequences were com- for 30 min while continuously rotated. All samples were finally pared with a local taxonomic reference library containing 2445 recovered in 3 mL elution buffer. sequences of 815 arctic (Sønstebø et al., 2010) and 835 boreal All PCRs were performed in an aDNA dedicated room at the (Willerslev et al., 2014) vascular taxa as well as 455 bryophytes Laboratoire d’ECologie Alpine, University Grenoble Alpes, using (Soininen et al., 2015), and assigned to the relevant taxon. Using the g and h universal plant primers for the short and variable P6 the local reference library confers the advantage of a more accu- loop region of the chloroplast trnL (UAA) intron (Taberlet et al., rate match with species that are found in the local environment. As 2007), and including a unique 8 bp long flanking sequence (tag) at almost all vascular plant species in Svalbard and the majority of the 5′ end to allow parallel sequencing of multiple samples (Bin- those found in neighbouring territories, as well as the circum- laden et al., 2007; Valentini et al., 2009). arctic region, are in the local reference library, we prioritized DNA amplifications were carried out in 50 µL final volumes matches against this database. On the other hand, if taxa were lack- containing 5 µL of DNA sample, 2 U of AmpliTaq Gold® DNA ing in the local library, there may have been no assignment, or an Polymerase (Life Technologies, Carlsbad, CA, USA), 15 mM erroneous one. Therefore, we also made comparisons with a sec-

Tris-HCl, 50 mM KCl, 2.5 mM MgCl2, 0.2 mM each dNTP, ond reference library generated after running ecopcr on the global 0.2 µM each primer and 8 µg Bovine Serum Albumin. One PCR EMBL database (release r117 from October 2013). Sequences negative control was carried out. All PCR samples (DNA and assigned to non-native taxa were blasted to check for potential controls) were randomly placed on PCR plates. Following the wrong assignments (http://www.ncbi.nlm.nih.gov/blast/). enzyme activation step (10 min at 95°C), PCR mixtures under- Extreme caution must be taken before accepting a taxonomic went 45 cycles of 30 s at 95°C, 30 s at 50°C and 1 min at 72°C, assignment in an environmental sample (Pedersen et al., 2015). plus a final elongation step (7 min at 72°C). Four individually Accordingly, to avoid any misidentifications, only sequences tagged PCR repeats were made for each sample to increase the matching 100% to reference library entries and occurring as at chance of detecting taxa represented by low quantities of DNA, as least 10 reads per PCR repeat were kept. The following were also well as to increase confidence in the taxa identified (Ficetola removed: (1) sequences having higher frequencies in negative et al., 2015). Equal volumes of PCR products were mixed (15 µL controls than in samples, (2) sequences occurring in <3 repeats in of each), and 10 aliquots of 100 µL of the resulting mix were then total (i.e. across all samples), (3) sequences belonging to food purified using MinElute Purification kit (Qiagen GmbH, Hilden, plants and thus suspected to be contaminants, and (4) sequences Germany). Purified products were then pooled together before suspected to be droplet contaminants or overflow from samples sequencing; 2 × 100 + 7 paired-end sequencing was performed from another study run at the same time. One complete sample, on an Illumina HiSeq 2500 platform using TruSeq SBS Kit v3 which appeared as an outlier in terms of low number of reads and (FASTERIS SA, Switzerland). repeats, was excluded (Supplementary Table S1, available online). By applying these thresholds, rare taxa were possibly missed but DNA sequences analysis and filtering potential errors were removed. In four cases, two sequences were assigned to the same taxon and combined (Saxifraga oppositifolia, Sequence data were analysed using the OBITools software pack- Micranthes, Pedicularis and Dicranaceae). For sequences that age (http://metabarcoding.org/obitools/doc/index.html). First, matched several taxa or were assigned to genus level only, the direct and corresponding reverse reads were assembled using likely taxa based on the current native flora of Svalbard were listed illuminapairedend, and sequences having a low alignment as potential species. Taxa identified in more than two of the four quality score (threshold set at 40) were filtered out (Supplemen- PCR repeats per sample were assumed to be certain, whereas the tary Table S1, available online). The retained reads were assigned validity of taxa found in fewer repeats was assessed carefully. to relevant samples using ngsfilter, keeping sequences matching 100% with tags and allowing a maximum of three mismatches with primers (Bienert et al., 2012). Strictly identical sequences Plant macrofossil analysis were then merged together (dereplication) using obiuniq, keeping Macrofossils were analysed for direct comparison with the information on their distribution among samples. All sequences sedaDNA record as well as for biostratigraphic correlation with

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Figure 2. Age–depth model for Lake Skartjørna, Svalbard. The calibrated 14C dates are shown in blue, and the lines show the age–depth curve (darker grey indicates more likely calendar ages, grey stippled line shows 95% confidence interval and red line shows best model based on weighted average of the mean). the record of Birks (1991). In total, 36 samples were collected con- peaks in Si and K, together with lower S and percent LOI, suggest tiguously as 4–6 cm slices. Sediment volume was determined by sedimentation dominated by terrigenous minerogenic input. The water displacement (100 mL used). Sample volumes were approx- upper unit boundary is defined by the top of the uppermost silty imately 50–60 cm3 (cf. Birks (1991), c. 160 cm3). Prior to sieving bed and a marked drop in Ca. Unit L1 likely reflects alternation each sample, c. 10 g sodium pyrophosphate (Na4P2O7 × 10H2O) between low-energy hydrologic conditions with minerogenic was added, mixed and left for at least 1 h to disaggregate clay deposition from suspension (clay), interrupted by phases of materials. For some samples, this step was repeated. Samples were enhanced slope input from the catchment. sieved using 250 µm meshes and plant macrofossils were identi- fied using a binocular microscope and based on the reference L2 (455–387 cm depth, 6900–5500 cal. BP). A transition to more collection at the herbarium TROM. The analyses were not intended regularly alternating 2–5 mm thick clayey laminae occurs at to be exhaustive and no attempt was made to identify bryophytes. 455 cm. Si and K are lower and their fluctuations are smaller. This suggests more stable sedimentation, mainly from suspension, and less variable influx of minerogenic sediments, probably because Results of reduced runoff from the catchment. The organic contribution to Chronology and lithostratigraphy the sediments as shown by the percent LOI is the highest of any unit (average LOI 8.3% as compared with average 7.3% of all All 12 AMS radiocarbon dates returned plausible ages spanning samples below and average 5.6% of all samples above). Also, the 7740 ± 40 to 1305 ± 30 14C years, corresponding to a calibrated S and Fe are high, likely indicating higher organic input and/or a weighted-mean range of 8477–1238 cal. BP (Table 1). The age– change in redox conditions. depth model revealed a fairly even sedimentation overall, although periods of lower sedimentation rate occurred around 530–390 cm L3 (387–315 cm depth, 5500–4600 cal. BP). A distinct drop in depth (8500–5500 cal. BP) and 260–210 cm depth (3800–2800 LOI to ca 5% and abrupt increases in Si, K and sedimentation rate cal. BP, Figure 2). Five lithostratigraphic units (L1–L5) were iden- define the base of unit L3. The unit is dominated by 1- to 3-mm- tified, based on sediment structure (Supplementary Figure S1, thick laminae interrupted by successions of coarser beds up to available online), geochemical and lithological compositions and 50–100 mm thick. There are large fluctuations in Si and K, with organic content (Figure 3). The elements Si, K and Ca are assumed peaks also associated with the reappearance of Ca. These varia- to characterize relative changes in input of terrigenous sediments tions suggest a reversion to sedimentation driven by relatively derived from the local metamorphic bedrock. Variations in sulphur high-energy terrigenous inputs. appear to track changes in organic content, and changes in Fe likely relate to redox variation. The presence of the delta close to L4 (315–165 cm depth, 4600–2300 cal. BP). The unit is domi- the northern part of the basin strongly suggests that the main ter- nated by clayey beds. The thicker silty beds characterizing units L1 rigenous sediment source has been direct input from the northern and L3 are absent. At the base of the unit is a small but distinct slopes rather than the drainage from Tjørnskaret pass. Aeolian increase in LOI to about 6%; Si and K fluctuate less than in L3 and input is regarded as a minor sediment source, as the catchment increase gradually from 227 cm depth. The more fine-grained char- area wind speeds are relatively low. Consequently, the changes in acter of the sediments, the absence of the thick silty laminae and the minerogenic content can be regarded as a proxy for changes in increase in the percent LOI indicate sedimentation mainly from slope stability and erosion from the catchment through time. suspension, probably with less slope-derived influx to the basin.

L1 (531–455 cm depth, 8600–6900 cal. BP). The unit is domi- L5 (165–60 cm depth, 2300–1100 cal. BP). At 165 cm, a transi- nated by alternating clayey laminae (5–10 mm) and silty beds (up tion to more weakly laminated sediment occurs. The lower to 40 mm thick), which are reflected in frequent (reciprocal) fluc- boundary is also characterized by a distinct temporary drop in Si tuations in percent LOI and the elements Si, K, Fe and S (Figure 3). and K, and the start of synchronous, larger amplitude fluctuations The thin laminae are characterized by higher LOI (up to 10%), of these elements. The percent LOI is generally low. Periodic high S and lower Si and K ratios. In the thicker, silty strata, distinct highs in Si and K may indicate episodes of higher influx of

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Figure 3. Sediment properties of the core from Lake Skartjørna, Svalbard. Black dots (•) indicate the depth of 14C-samples. Lithostratigraphic units (L1–L5) are marked. Water content is given as percentage wet weight. Loss-on-ignition (LOI) is given as percentage dry weight. Selected elements analysed by XRF are given as ratio to Ti (see methods). Filled areas mark values above the mean. Grain size fractions are given for clay (<2 µm), fine silt (2–8 µm) medium silt (8–16 µ), coarse silt (16–63 µm) and sand (63–2000 µm) as percentages of total volume. The lithology show the relative frequency and thickness of laminae (vertical bars) and the presence of coarser beds (dotted areas), for high resolution photo see electronic (Supplementary Figure S1, available online). terrigenous sediments to the lake from the catchment, but they are they are interpreted with caution (Table 2, Figure 4). In total, 34 not evident as major textural changes. taxa of 13 families of vascular plant were identified, 32 (94%) determined to genus level and 19 (56%) to species level (when assuming a correct match to local taxa). In addition to the vascular sedaDNA plants for which the primers are designed, we also detected 12 We obtained 13,030,207 reads of 62,568 unique sequences bryophytes and two algal taxa (Table 2). assigned to 40 samples (Supplementary Table S1, available Salicaceae, Oxyria digyna, Bistorta vivipara, Micranthes, online). After subsequent filtering, 53 taxa (9,197,220 reads) Papaver and Saxifraga oppositifolia were present in nearly all remained, of which 48 (9,191,407 reads) were assumed to be of PCR repeats of all samples (Figure 4). Further taxa present in local origin, whereas five taxa (5813 reads) were exotics (Table 2, most samples, but with lower repeats, were the herbs Pedicularis, S2). Of the 48 local taxa, 27 taxa were recorded in three or Silene acaulis, Cerastium, Draba and the rush Luzula. The major- more repeats, and a further five were confirmed by macrofossils ity of taxa identified are temperature-indifferent or weakly ther- (Table 2). The identities of these 32 taxa were assumed certain. A mophilous in Svalbard and common in the northern arctic tundra further 16 were found in only one or two out of four PCR repeats and polar desert zones (Table 2). Three taxa that are only present in a sample, many likely the same as identified macrofossils, but in climatically more favourable sites in Svalbard today, Arabis

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Table 2. All taxa recorded from Lake Skartjørna from sedaDNA (this study) and macrofossils (1 is this study, 2 is Birks, 1991).

Family Taxa sedaDNA Taxa macrofossils Max. repeats Sum repeats Sum reads Birks n Therm. Zone

Betulaceae Betula (nana ssp. tundrarum) 2 5 786 I C(r) Brassicaceae Brassicaceae undiff.2 5 Brassicaceae cf. Braya glabella ssp. 1 IV A(r) purpurascens2 Brassicaceae Arabis alpina Arabis alpina1,2 2 5 223 1 II B(r) Brassicaceae Cardamine (bellidifolia) 2 4 218 V A(f) Brassicaceae Cochlearia (groenlandica) 2 8 524 V A(f) Brassicaceae Draba (13 species) Draba2 3 47 5,211 2 Caryophyllaceae Caryophyllaceae1,2 1 Caryophyllaceae Cerastium (arcticum, alpinum, Cerastium arcticum/ 4 44 2,507 1 regelii) Cerastium alpinum2 Caryophyllaceae Minuartia (rubella) Minuartia rubella2 1 3 86 1 V A(f) Caryophyllaceae Sagina (nivalis, cespitosa) Sagina nivalis2 1 3 109 2 V A(r) Caryophyllaceae Silene (uralensis, involucrata) 1 3 112 Caryophyllaceae Silene acaulis (ssp. acaulis) Silene acaulis1,2 4 47 15,824 6 IV A(s) Ericaceae Harrimanella hypnoides2 1 II C(r) Juncaceae Juncus biglumis Juncus2 1 3 50 5 V A(f) Juncaceae Luzula (arcuata, confusa, nivalis, Luzula2 4 77 10,584 11 wahlenbergii) Lycopodiaceae Lycopodiaceae (Huperzia arctica) 2 4 84 III B(s) Orobanchaceae Pedicularis (hirsuta, dasyantha) 4 43 1,759 Papaveraceae Papaver (dahlianum, Papaver2 4 120 29,857 7 cornwallisense) Poaceae Agrostidinae (Calamagrostis neglecta) 2 4 164 II C(s) Poaceae Festuca (rubra, baffinensis, 4 23 2,148 hyperborea, edlundiae, brachyphylla) Poaceae (Phippsia algida, P. concinna, Hierochloë 2 5 460 alpina) Poaceae Deschampsia (alpina, 3 9 298 sukatschewii) Poaceae Poeae (Poa alpina var. alpina, P. 4 38 2107 alpina var. vivipara) Poaceae Puccinellia (7 species) 3 12 612 Polygonaceae Bistorta vivipara Bistorta vivipara1,2 4 156 406,904 25 V A(f) Polygonaceae Koenigia islandica 3 6 154 III B(r) Polygonaceae Oxyria digyna Oxyria digyna1,2 4 155 77,373 4 V A(f) Ranunculaceae Ranunculaceae (10 species) 4 30 1,434 Ranunculaceae Ranunculus pygmaeus Ranunculus pygmaeus2 1 3 78 1 IV A(f) Rosaceae Dryas (octopetala) Dryas1,2 4 111 38,488 17 IV B(r) Salicaceae Salicaceae (Salix herbacea, S. 4 156 7,638,215 36 polaris, S. reticulata, S. glauca) Salicaceae Salix cf. glauca2 1 I C(r) Salicaceae Salix herbacea2 3 II B(r) Salicaceae Salix reticulata1,2 4 II C(s) Salicaceae Salix polaris1,2 36 V A(r) Saxifragaceae Saxifraga (aizoides) Saxifraga aizoides1,2 4 136 27,977 6 III B(s) Saxifragaceae Saxifraga (cernua, rivularis, 4 99 17,072 hyperborea, cf. svalbardensis) Saxifragaceae Saxifraga rivularis2 14 n.d. A(r)

(Continued)

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Table 2. (Continued)

Family Taxa sedaDNA Taxa macrofossils Max. repeats Sum repeats Sum reads Birks n Therm. Zone

Saxifragaceae Saxifraga cernua2 1 V A(f) Saxifragaceae Saxifraga cespitosa (ssp. cespitosa) Saxifraga cespitosa/ 4 91 6,825 10 V A(f) Saxifraga platysepala2 Saxifragaceae Saxifraga hirculus (ssp. compacta) 1 3 276 IV A(s) Saxifragaceae Saxifraga oppositifolia Saxifraga oppositifolia1,2 4 156 885,423 35 V A(f) Saxifragaceae Saxifragaceae (Micranthes Micranthes nivalis/ 4 93 8,872 1 nivalis, M. tenuis, M. hieracifolia, Micranthes tenuis2 M. foliolosa) Algae Closteriaceae Closterium littorale 3 4 107 N/A Desmidiaceae Cosmarium botrytis 3 19 687 N/A Bryophytes Andreaeaceae Andreaea rupestris 4 53 2,323 N/A Bartramiaceae Bartramiaceae 1 3 122 N/A Bryaceae Bryum 3 13 430 N/A Bryaceae Bryaceae 2 6 146 N/A Dicranaceae Dicranaceae 2 9 283 N/A Ditrichaceae Distichium (capillaceum, hagenii, 1 4 90 N/A inclinatum) Encalyptaceae Encalypta alpina 3 33 934 N/A Grimmiaceae Grimmiaceae 3 13 416 N/A  Rhabdowei- Arctoa (fulvella, cf. anderssonii) 2 9 339 N/A siaceae Seligeriaceae Blindia acuta 1 4 132 N/A Sphagnaceae Sphagnum 2 9 337 N/A Timmiaceae Timmia 2 9 322 N/A

Taxa only identified by one of the proxies are shown in blue (sedaDNA) and purple (macrofossils). Green indicates that the same taxa might have been detected with both methods; darker green indicates that the taxa were identified with a higher taxonomic resolution. Taxa identified by sedaDNA in minimum three of the four PCR repeats and/or confirmed by macrofossils are regarded certain identifications and shown in bold. For DNA sequences matching to several species, the Svalbard representatives are given in brackets. ‘Max repeats’ are the maximum number of PCR repeats within one sample, whereas ‘sum repeats’ are sum of PCR repeats where the taxa occur across all samples. ‘Birks n’ is number of samples where the taxa were found out of 36 samples analysed (Birks, 1991). Division of species into thermal groups (I = Strongly thermophilous, II = Distinctly thermophilous, III = Moderately thermophilous, IV = Weakly thermophilous, and V =T emperature indifferent) follows Elvebakk (1989). Nomenclature and northernmost bioclimatic zone (A = Polar desert zone, B = Northern arctic tundra zone, C = Middle arctic tundra zone) where the species occurs as rare (r), scattered (s) or frequent (f) follows the PanArctic Flora checklist (Elven et al., 2011). Thermal groups and bioclimatic zones are given for the taxa identified to the lowest taxonomic level; bold indicates thermal conditions warmer than present. alpina, Betula and Agrostinae, have scattered occurrences in one vascular plants and five taxa of bryophytes are found in this zone. to two repeats between 8600 and 1800 cal. BP. Three zones were This is a lower taxon count than the other two zones, but also the identified visually based on the DNA data (D1–D3), the two time span is shorter and the number of samples is lower. lowermost roughly corresponding to the lithostratigraphic units L1 and L2 (Figure 4). D3 (387–74 cm depth, 5500–1200 cal. BP). At the transition to this zone, Festuca and Ranunculaceae disappear (but the latter D1 (525–441 cm depth, 8500–6600 cal. BP). The zone is char- reappears at 340 cm), whereas nearly all species of bryophytes acterized by relatively high values of Festuca and Poa alpina, begin to appear regularly, including Blindia acuta and Arctoa, along with Ranunculaceae. Koenigia islandica and Lycopodia- which are rare in Svalbard today. Overall, this zone has the highest ceae are limited to this zone, which also features the richest grass diversity of vascular plants (32 of 34 taxa) and includes all 12 bryo- flora. Twenty-six of the 34 vascular plant taxa (76%) and six of phyte taxa. There is no clear biostratigraphic pattern in this zone. the 12 bryophyte taxa (50%) appear in this lowermost zone.

D2 (441–387 cm depth, 6600–5500 cal. BP). In this zone, Koe- Plant macrofossils nigia islandica disappears, whereas Dryas and Saxifraga cespi- Three zones (M1–M3) are apparent, but the zone boundaries tosa become more frequent. Ranunculaceae and Festuca are still are not well demarcated (Figure 5) and do not coincide with present. Also, there is a turnover of bryophytes, with four taxa the lithologic or sedaDNA zones. M1 mainly corresponds to disappearing and three new ones appearing; Encalypta alpina L1 and D1, whereas the M2/M3 boundary falls within D3 and appears regularly from this zone onwards. Twenty-three taxa of L4 (Figure 6).

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Figure 4. Results of sedaDNA analyses from Lake Skartjørna, Svalbard. The x-axis refers to number of PCR repeats with 10 or more reads. Only taxa with 100% match to reference database are included. Taxa with either >50% successful PCR repeats in minimum one sample and/or confirmation by macrofossil are regarded as certain taxa (black), whereas taxa found in lower numbers of repeats (white) should be inferred with some caution (see Table 2). Tentative zonation based on the DNA results are indicated (D1–D3). Saxifraga taxa abbreviated as cern. (cernua), riv. (rivularis) and hyp. (hyperborea).

Figure 5. Macrofossils found in Lake Skartjørna, Svalbard. The x-axis refers to number of occurrences in a total volume of 50–60 mL except for Nostoc pruniforme gelatinous spheres, which are presented on a relative scale: (1) few (1–5), (2) moderate (6–20) and (3) abundant (>20). All plant macrofossils are seeds unless otherwise stated. Analysed by P. Sjögren, 2015. Tentative zonation based on the macrofossil results is indicated (M1–M3).

M1 (529–434 cm depth, 8500–6400 cal. BP). The zone has the as Nostoc pruniforme gelatinous spheres. Single seeds of Arabis highest abundance and diversity of plant macrofossils. There is alpina and Brassicaceae undiff. were identified from the lower- high but variable abundance of Salix polaris leaves. Other fre- most two samples (529–521 cm depth, 8500–8400 cal. BP). Sam- quent macrofossils in this zone include Salix reticulata, Saxifraga ples with fewer macrofossils appear to correlate with the coarser oppositifolia, Saxifraga undiff. and Silene acaulis seeds, as well sedimentary beds of L1.

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Figure 6. Comparison of zones and selected proxies. Taxon richness sedaDNA – number of vascular plant taxa per sample with one or more PCR repeats with 10 or more reads. Taxon richness macrofossils – number of identified taxa per sample as given in Figure 5. Scales of sedaDNA results are in number of PCR repeats with 10 or more reads. Sand and coarse silt (16–2000 µm) is given as percentage volume of all fine fraction. Filled areas mark values above the mean.

M2 (434–291 cm depth, 6400–4200 cal. BP). There is a strong are similar. The most significant sedimentological shift reported drop in the overall concentration of macrofossils, whereas the by Holmgren et al. (2010), a major decline in organic carbon at richness is variable (Figures 5 and 6). This zone is characterized 415 cm depth (5400 cal. BP), can be correlated with the decline in by a relatively high abundance of Dryas octopetala leaves. The percent LOI at the L2/L3 boundary at 387 cm (5500 cal. BP) in abundance of Salix polaris leaves decreases; Salix reticulata the present core (Figure 3). leaves are present in the lowermost part but disappear above The chronology of the 335 cm core analysed by Birks (1991) 397 cm (5700 cal. BP). No Saxifraga undiff. seeds occur above rests on a single basal radiocarbon date (330–335 cm): 8110 ± 115 this zone. The zone displays a gradual decline in species richness 14C BP (9000 ± 400 cal. BP 2σ error interval), plus palaeomagnetic from 12 taxa to four. correlation. However, an additional unpublished date at 155– 160 cm of 2470 ± 80 14C BP (2550 ± 130 cal. BP) was done by G. M3 (291–63 cm depth, 4200–1100 cal. BP). This zone is defined Miller in 2005 (H. H. Birks and J. Mangerud, personal communi- by low abundance and richness of macrofossils. Only Salix polaris cation) and supports the palaeomagnetic correlation. Assuming leaves and Saxifraga oppositifolia seeds occur frequently. LOI is proportional to C, a major decline in organic carbon occurs, but earlier than in the core of Holmgren et al. (2010). Birks’ (1991) macrofossil record is also divided into three zones. The lowermost Correlation with other records from Skartjørna and uppermost zones have a similar composition to our M1 and The core lengths and time intervals covered in this study (530 cm; M3. However, in Birks’ record, the high concentration of Salix 8600 years) and those of Holmgren et al. (2010; 550 cm; 8200 years) polaris leaves is maintained throughout the middle zone, and the

Downloaded from hol.sagepub.com at Aberystwyth University on June 2, 2016 Alsos et al. 637 boundary between the middle and upper zones is based on a sub- transported as complexes with other particles (England et al., sequent decline at c. 150 cm (2600 cal. BP). The S. polaris leaves 2004; Taberlet et al., 2012). aside, it is possible that our M2/M3 boundary at 291 cm correlates with the change in Birks’ (1991) record at 190 cm depth, as Dryas The capacity of sedaDNA to provide a record of past octopetala leaves occur more frequently below both these bound- aries, which date to approximately 4000 cal. BP. Thus, we con- flora clude that the zones of Birks (1991) can be roughly correlated with The sedaDNA provides a coherent record that is, in some our zones M1–M3. instances, complementary to that of the plant macrofossils. For example, while sedaDNA has lower taxonomic resolution than macrofossils for the important family Salicaceae, it reveals taxa Comparison between sedaDNA and macrofossils that were not present as macrofossils, such as Huperzia and sev- All families recorded as macrofossils in either this study or the eral species within the Poaceae, taxa less commonly recorded in more extensive study by Birks (1991) were also found in the macrofossil studies. Previous sedaDNA studies from the Arctic sedaDNA (Table 2). In addition, sedaDNA identified Betulaceae are characterized by less overlap between sedaDNA and macro- (the only local species is Betula nana ssp. tundrarum), Lycopodia- fossils (10–35%: Jørgensen et al., 2012; Parducci et al., 2012a, ceae (only local species Huperzia arctica), Orobanchaceae (identi- 2012b, 2013, 2015; Pedersen et al., 2013; 56%: Porter et al., fied to Pedicularis), Poaceae (Poa alpina identified to species, 2013). Our higher taxonomic recovery from sedaDNA likely Deschampsia, Festuca and Puccinellia identified to genera, one reflects the almost complete reference library available, and pos- sequence identified to Phippsia or Hierochloë, and Agrostidinae, sibly also the quantity of sediment used for extraction and the assigned to its only local representative, Calamagrostis neglecta). PCR and sequencing conditions. Only 27 of 48 taxa occurred in At lower taxonomic levels, direct comparison is not always all four PCR repeats, suggesting that some taxa would be missed possible. For example, Brassicaceae identified in macrofossils in studies that used fewer repeats. This argues for using multiple could correspond to any or none of the four Brassicaceae taxa repeats to detect species with low concentrations of DNA, even identified with sedaDNA (Table 2). However, all taxa found in though it may increase the occurrence of false positives (Ficetola more than one sample of macrofossils were identified with high et al., 2015). However, in our case, single repeats seemed to be certainty in the sedaDNA analyses (except Juncus). All but two reliable as using this threshold increased the number of local taxa genera identified as macrofossils (cf. Braya glabella ssp. pur- by 16 (Table 2) but added only one new exotic taxon (Convallaria purascens and Harrimanella hypnoides) were identified with majalis, Supplementary Table S2, available online). Overall, the sedaDNA, although some occurred in rather few PCR repeats large degree of overlap in species composition between sedaDNA (Table 2). Other taxa are only detected by sedaDNA, for example, and macrofossils (Table 2) confirms that sedaDNA may be a reli- Koenigia islandica, Pedicularis and different species of Poaceae able approach to reconstructing past vegetation. (Figure 4–6, Table 2). Overall, 34 and 28 taxa of vascular plants All but one of the taxa found as macrofossils in more than one were identified with sedaDNA and macrofossils, respectively. In sample were identified with a conservative evaluation of the many cases, the taxa were identified to species level with both sedaDNA results (i.e. three or more PCR repeats). These taxa are methods. Macrofossils were superior in distinguishing species of widespread in Svalbard today (Alsos et al., 2015), most are typi- Salicaceae. The numbers of taxa detected per sample were much cally vegetation dominants and they produce a relatively high higher for sedaDNA (mean ± SE = 15.9 ± 0.42) than for macrofos- biomass (e.g. Silene acaulis, Luzula spp., Dryas, Salix polaris, sils (2.37 ± 0.19 and 5.83 ± 0.57 for this study and that of Birks Saxifraga oppositifolia; Elvebakk, 1985, 2005), and they are cur- (1991), respectively; Figure 6 and Supplementary Figure S2, rently common in the catchment vegetation. The high abundance available online). of macrofossils of Salix polaris and Saxifraga oppositifolia, for In our record, the pattern of declining macrofossil taxon rich- example, indicate relative high abundance in the catchment veg- ness with time follows that of the sedaDNA, but with a much etation. DNA of several taxa with a generally more scattered lower number of taxa overall (Figure 6). A similar decline is also occurrence today (e.g. Minuartia, Juncus biglumis) or at their observed in study of Birks (1991, Supplementary Figure S2, thermal limit (Betula, Arabis alpina) was also identified, although available online). Dominant taxa such as Salicaceae, Bistorta in less than three of four PCR repeats (Table 2). This suggests that vivipara and Saxifraga oppositifolia are represented in all sam- the positive correlation between biomass in vegetation and recov- ples of both proxies. For most taxa identified with both proxies ery in DNA found in modern soil samples (Yoccoz et al., 2012) (Oxyria digyna, Saxifraga aizoides, Papaver, Arabis alpina, Min- may also apply to fossil samples from lake sediments. While Yoc- uartia, Sagina, Ranunculus pygmaeus, Dryas octopetala, Ceras- coz et al. (2012) used number of reads as quantification of plant tium and Draba), sedaDNA detects them in more samples than do abundance in their modern soil samples, we used number of PCR the macrofossils (compare Figure 4 with Figure 5 and Birks, repeats (0–4), which is a more conservative interpretation more 1991). For example, while macrofossils of D. octopetala only appropriate for aDNA. were found in nine samples, it was detected in 34 samples of Variation in macrofossil abundance can often be interpreted sedaDNA (Figure 6). as changes in past plant abundances (Birks, 2003, 2014), but infrequent occurrences of macrofossil taxa likely reflect changes in source area (e.g. fluvial input vs slope input) and a degree of Discussion serendipity in detection, with absence particularly hard to inter- pret. For example, macrofossils of Dryas octopetala are rarely This new, well-dated record and the previous detailed plant mac- recorded in the top zone of both macrofossil records, whereas rofossil study of Birks (1991) together provide an excellent Dryas is present in most sedaDNA samples (Figure 6). Here, the opportunity to assess how sedaDNA augments and/or modifies molecular approach may be superior, as it exhibits more consis- interpretations of past flora and climate in high-arctic settings. tent detection of taxa. The plant material contributing to the sedaDNA is likely derived from overland flow from melting snow beds or from rain events, erosion of material by small streams or at the lake shore and via Holocene environmental change at Lake Skartjørna wind deposition (possibly from more distant sources; Birks, 1991; The set of proxies retrieved from Lake Skartjørna gives insight Birks et al., 2004; Glaser, 1981). These are essentially the same into past environmental conditions, but many of the signals are sources as macrofossils, but sedaDNA is possibly not represented subtle, indicating moderate environmental changes. The observed in the same proportions. In addition, DNA from soil may be changes are discussed here primarily in conjunction with two other

Downloaded from hol.sagepub.com at Aberystwyth University on June 2, 2016 638 The Holocene 26(4) records from Lake Skartjørna (Birks, 1991; Holmgren et al. 2010). cf. glauca (sporadic to ~4000 and ~2500 cal. BP, respectively). Sediment properties can be heterogeneous across lake basins for a All the above-mentioned taxa (except Salix cf. glauca, which is variety of reasons, and not all the proxy records agree. We assume extinct) are all classified as strongly or distinctly thermophilous that the greater core depth retrieved in the current study as well as in Svalbard today and have a northern limit in the Middle arctic the one by Holmgren et al. (2010) reflect higher influx from tundra zone (Table 2); the records at Skjartørna lie outside their the erosion of the northern slope, whereas the shorter one collected modern range limits (as much as 30 km outside for Arabis alpina by Birks (1991) might be closer to the axial inflow (Figure 1). (nearest site Diabasbukta) and Betula (presumably Betula nana, However, the main patterns in all the datasets suggest a warm early nearest site Colesdalen; Alsos et al., 2015). All the above species Holocene and a subsequent cooling trend until the present. also require consistent winter snow cover. Thus, the combined vegetation records, and our lithologic record, are consistent with enhanced summer warmth and relatively high levels of precipi- Early and mid-Holocene (c. 8600–5400 cal. BP; tation. Mean July temperatures may have corresponded to 560–375 cm) Middle arctic tundra zone values (minimum 6°C, Elvebakk, In the early part of our record (8600–7000 cal. BP), the high-fre- 2005; Walker et al., 2005), that is, 1–2°C warmer than today. quency variation in %LOI reflects the alternation of bands of silty Other records show increased local pollen production approxi- material and more organic, fine-grained sediment, suggesting an mately 8000–5200 cal. BP (Hyvärinen, 1968, 1969, 1970), and episodic and dynamic runoff regime to the lake, possibly accentu- peat formation during the period 8800–4200 cal. BP also sug- ated in the sediment record by the relative proximity of the coring gests a warmer climate (Göttlich and Hornburg, 1982; van der site to the northern slope. The maximum organic content, as Knaap, 1988, 1989). expressed by percent LOI, is relatively high for a high-arctic lake The sedaDNA record after ~6400 cal. BP contains taxa toler- (~10%). Values decline ~5500 cal. BP to 4–5%. A similar overall ant of dry conditions (e.g. Dryas, Andreaea, Encalypta alpina). trend in carbon content is recorded by Holmgren et al. (2010): The increase in Dryas is also clearly seen in the macrofossil ~3% in the early record dropping to ~1.5% after ~5400 cal. BP. In record. At about the same time, the lithostratigraphic record sug- our record, Nostoc, which fixes atmospheric nitrogen, is continu- gests reduced and/or less variable runoff. Overall macrofossil ally present from 8600–6500 cal. BP. Holmgren et al. (2010) input to the lake dropped dramatically, also ~6400 cal. BP (Figure report high diatom concentrations between 8100 and 6600 cal. BP, 6). The changes in terrestrial vegetation composition, macrofossil interpreted as relatively high overall algal productivity. Further- abundance and lithology likely reflect a change in precipitation, more, they record low (<10) C:N ratios for this period, which indi- such as an overall reduction in winter snow cover that favoured cate dominance of autochthonous over allochthonous sources. the development of Dryas heaths on open slope and tops. Thus, the most likely explanation for the relatively high levels of organic content observed in the early part of the two records is higher biologic productivity driven by warmer growing-season Mid- and late Holocene (5400–c. 1000 cal. BP; temperatures and/or a longer ice-free period. The distinct drop in 375–60 cm) percentage LOI around 5500 cal. BP co-occurs with the emergence The sediment record (particularly lithology, K, Si and Ca) sug- of silty beds in the sediment and an increased sedimentation rate, gest that the magnitude and variability of runoff increased and the %LOI values could have been suppressed by increased between 5600 and 4600 cal. BP. More stable sedimentation minerogenic inflow. High biological productivity as perceived by characterized the period 4600–2300 cal. BP and then was fol- the %LOI record could thus have continued for some time, and/or lowed by stronger pulses of minerogenic input between 2300 the decline may have been more subtle. The overall higher concen- and 1100 cal. BP. After 4200 cal. BP, macrofossil plant species tration and diversity of macrofossils found in our and Birks (1991) richness per sample is generally low (Figure 6, Supplementary records, the higher diversity of plants found in the sedaDNA, and Figure S2, available online). However, the sedaDNA data show the occurrence of relatively thermophilous species in all three that all taxa except three vascular plants and one bryophyte records also indicate that a more lush terrestrial flora was present. persisted after 4200 cal. BP. Thus, most taxa recorded from the High levels of plant macrofossil delivery to the lake occurred early Holocene survived locally. Drier and more open condi- between 8500 and ~7000 cal. BP, declining by 6500 cal. BP to tions are suggested by consistent presence of Dryas and Draba lower values that persist through the remaining record. For much and the bryophytes Andreaea and Timmia. The catchment prob- of this time (until ~7000 cal. BP), strong banding of the sediments ably supported a geomorphologically and aspect-controlled suggests multiple high runoff events, which could have entrained mosaic of communities, including Dryas octopetala heath, plant material. The runoff events may have been intense but short- open herb communities and moist snow-bed and drainage com- lived with much of the influx of plant material occurring gradu- munities, as it does today. ally between events, suggesting relatively high vegetation cover. The find of a Chara oospore at about 3500 cal. BP is remark- Birks (1991) also reports relatively high concentrations early in able. Today, it is found only in warm springs on Spitsbergen the record, but they persist to ~2500 cal. BP. The difference in the (Chara canescens; Langangen, 2000). The single find in records may be largely because of proximity to the axial influx Skartjørna is morphologically similar to Chara contraria, occur- versus erosion of the northern slope as these sites have differences ring in northern Norway, might originate from long-distance in vegetation. Given the difference between the records and the transport by birds, particularly geese (Langangen, 2000, personal possibility of local sedimentary variations, we recommend cau- communication). tion in interpreting environmental change based on this change The changes at Skartjørna that may signal sparser vegetation (or lack of change) in macrofossil concentrations. and lower biomass 5500–4000 cal. BP reflect other inferred The DNA record includes three taxa indicative of relative changes in the marine and terrestrial environments. Sea-surface warmth (Arabis alpina, Agrostidinae and Betula nana), which temperatures started to decline c. 7000–5000 cal. BP, and further first occur early (8500–6500 cal. BP) and reappear sporadically cooling began around 4000 cal. BP (Rasmussen et al., 2012). On through the record until ~1500 cal. BP. Arabis alpina was also land, the nearby glacier Linnébreen re-formed around 4600 cal. found as a macrofossil both in our and Birks’ record. In addition, BP and advanced c. 2800 and 2400 cal. BP (Reusche et al., 2014; Birks (1991) recorded several other thermophilous species: Svendsen and Mangerud, 1997). Overall, these observations are Harrimanella hypnoides (synonym Cassiope hypnoides, early consistent with the onset of the Neoglaciation c. 5000–4000 cal. Holocene only, ~9000–8000 cal. BP), Salix herbacea and Salix BP in the North Atlantic region (Miller et al., 2010).

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Resilience of tundra communities in the face of in distinguishing some taxa, for example Salix ssp., whereas climate change sedaDNA was superior in detecting Poaceae taxa. Using the num- A striking feature of the molecular record is that there has been ber of repeats as a basic estimate of abundance, sedaDNA reflects little floristic turnover in the local vegetation through the Holo- the higher biomass of common arctic taxa, and these are identi- cene, despite a decrease in vegetation productivity inferred from fied with high certainty. As with macrofossils, the likelihood of the macrofossils. This provides firm evidence in support of Birks’ detection is probably related to abundance in the vegetation. (1991) conjecture that was based on macrofossils alone. Even However, more taxa were detected with sedaDNA than with mac- some thermophiles persisted (but only a single repeat of the ther- rofossil analysis, suggesting that it is more sensitive in detecting mophilic species Arabis alpina is recorded after 5700 cal. BP). less abundant taxa. The Skartjørna record corroborates other stud- The scattered occurrence of Betula until 1800 cal. BP, long after ies in that its record of thermophilic species indicates tempera- cooler conditions were established, may reflect its ability to sur- tures 1–2°C higher than present on Spitsbergen during the early vive by clonal growth under conditions too cold for sexual recruit- part of the Holocene. In our record, the main environmental ment (Alsos et al., 2002, 2003). The combined proxy records changes occurred c. 7000–5500 cal. BP, as either gradual or step- suggest the gradual attrition of suitable habitats for thermophiles wise shifts in temperature, precipitation regime, terrestrial eco- in response to cooling and drying of the climate, but nevertheless, systems and lake sedimentation. The molecular data indicate that survival of most taxa in situ. This may be related to fine-scale even species with highly intermittent occurrence as macrofossils heterogeneity of the landscape that supports a range of microcli- persisted throughout most of the study period. We might expect mates (Armbruster et al., 2007) that buffered against the overall that thermophilous species that are currently highly restricted on lowering of temperature by 1–2°C. Spitsbergen will expand again (assuming sufficient precipitation) The Skartjørna plant records (this study, Birks, 1991) show that and that both terrestrial and lacustrine productivity will increase. thermophilic species had broader distributions on Spitsbergen in However, as future warming is likely to reach 2–4°C, we may also the early Holocene. This is consistent with early Holocene range see responses that cannot be anticipated by reference to the avail- extensions of thermophilic species in the southernmost island of able Holocene records. Svalbard (Bjørnøya, Wohlfarth et al., 1995), East Greenland and Acknowledgements northern Eurasia (Bennike et al., 1999; Binney et al., 2009), and also range expansion of Betula nana in Svalbard in the early Holo- We thank the University Centre in Svalbard for logistic support, cene (Andersson, 1910). Based on this history, we might expect Henrik Rasmussen for field assistance, Frédéric Boyer for help future warming of 1–2°C mean July temperature to drive an with DNA raw data handling, Tom Bishop and Peter Langdon increase in cover and productivity and the expansion of local ther- for help with depth-age model, and Ingvild Hald for grain-size mophilic species (given sufficient precipitation and conditions con- analyses. We also thank Hilary Birks and an anonymous reviewer ducive to establishment), but not major floristic change. However, for very constructive comments. future warming is likely to reach at least 2–4°C mean July tempera- ture above present; this temperature increase is unprecedented in Declaration of conflicting interests the Holocene and may see the appearance of new elements in the Ludovic Gielly is one of the co-inventors of patents related to g-h flora, assuming effective dispersal and establishment (Alsos et al., primers and the subsequent use of the P6 loop of the chloroplast 2007). trnL (UAA) intron for plant identification using degraded tem- plate DNA. These patents only restrict commercial applications and have no impact on the use of this locus by academic Added understanding of vegetation from sedaDNA researchers. compared with only macrofossils Because of the large degree of concurrence of taxa detected as Funding sedaDNA and macrofossils (Table 2), one may argue that the The work was support by the Research Council of Norway (grant proxies show overlap rather than being complimentary as sug- nos. 213692/F20 and 230617/E10 to Alsos). gested by others (Jørgensen et al., 2012; Parducci et al., 2013, 2015; Pedersen et al., 2013). However, the timing of change based References on sedaDNA differs from that of macrofossils (Figure 6 and Sup- Alsos IG, Arnesen G, Sandbakk BE et al. (2015) The flora of plementary Figure S2, available online), indicating that the prox- Svalbard. Available at: http://svalbardflora.no. ies pick up different signals of change; indeed, the majority of Alsos IG, Eidesen PB, Ehrich D et al. (2007) Frequent long- taxa changing around 6600 and 5500 were only recorded in distance colonization in the changing Arctic. Science 316: sedaDNA. A more important contribution of the sedaDNA from 1606–1609. this site is the observation that most taxa persisted throughout the Alsos IG, Engelskjøn T and Brochmann C (2002) Conservation period studied even though they are only found in scattered parts genetics and population history of Betula nana, Vaccinium of the period as macrofossils. For example, only one and two uliginosum, and Campanula rotundifolia in the arctic archi- macrofossils of Cerastium and Draba were found, respectively pelago of Svalbard. Arctic, Antarctic, and Alpine Research (Birks, 1991), whereas they were recorded in most sedaDNA 34: 408–418. samples (Figure 4). 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